By that same theory, though, the executive doesn't have to respect that constitutionality.
So they pass the bill expanding the Supreme Court, and included in the bill is a provision that strips jurisdiction for any court to evaluate it. If they try to exercise their jurisdiction anyway, the executive says "suggestion noted" and puts people on the court anyway.
This is a major weakness in our constitution. One of many, many weaknesses.
In general, separation of powers has turned into an incredibly outdated idea with costs that are far greater than the benefits. It was such a product of 18th century thinking. Of course it was separated into three -- almost everything came in groups of three back then. And of course it was assumed to be a nice tidy separation. They were drawing on abstract musings with little connection to the world, and of course as the world got more complex, the separation became even less workable.
Basically our constitution is simultaneously a) one of the great achievements in world history; and b) an institution that has outlived its usefulness and needs to be replaced.